“Amplifiers” are any devices, circuits or systems capable of converting an input signal into an output signal with an increased energy level. Amplifier circuits and systems are used in many different applications. Conventional mobile phones, for example, commonly incorporate power amplifier systems to amplify radio frequency (RF) signals that are transmitted from the phone to a base station or other receiver. Generally, amplifier systems include any number of transistors or other non-linear gain elements that are capable of increasing an electrical current or voltage representing the input signal. Certain types of transistors that have experienced widespread use in many amplifier systems include the bipolar junction transistor (BJT) and the hetero-junction bipolar transistor (HBT). Many different types of transistors have been formed in silicon, silicon germanium (SiGe), gallium arsenide (GaAs) and other materials, as is well understood in the art.
As a skilled radio frequency (RF) designer can attest, maximal power transfer from an amplifier (or other source of power) to a load generally occurs when the impedances of the source and load match. If impedances do not match, some of the power produced by the source can be reflected back to the source rather than being delivered to the load. Hence, it is generally desirable to match the impedance of a source (e.g., a power amplifier) to the impedance of a load (e.g. an RF antenna). Impedance matching can be complicated, however, by environmental effects such as the presence of nearby conductive objects that can create standing electromagnetic waves. One measure of system performance, then, is the voltage standing wave ratio (VSWR), which is related to the ratio of the power sent forward from the amplifier to the amount of power that is reflected back from the antenna or other load due to standing waves and the like.
Conventional silicon BJTs and HBTs generally have not been widely deployed as power amplifiers used in wireless telephony due to well-known ruggedness issues related to high VSWR conditions. In particular, conventional silicon BJT and HBT devices have been known to generate “avalanche” currents in response to relatively high VSWR conditions that can occur in wireless communication environments. Avalanche currents typically result when an electric field (such as the field produced in an RF transmitter) accelerates free electrons in a semi-conducting material; the accelerated electrons, in turn, can accelerate other electrons, ultimately leading to rapidly increasing currents in undesired pathways. These parasitic currents, in extreme cases, could lead to destruction of the transistor, thereby rendering the power amplifier (and the telephone or other device) inoperable. Although avalanche currents typically occur at lower voltages in silicon devices, they are also know to occur in GaAs and other devices as well.
Many attempts have been made to address the ruggedness issues associated with BJT and HBT amplifiers operated in high VSWR environments. Generally, engineers have attempted to avoid avalanche conditions entirely by controlling the output of the amplifier. This may be accomplished, for example, with suitable ballast circuitry and/or diode clamping, or by simply detecting when VSWR issues may occur and reducing power to the amplifier at such times. Each of these schemes has met with limited success, at best. At the very least, none of the techniques attempted in the past address the root cause of the ruggedness issues, so their usefulness has been somewhat limited. Hence, silicon bipolar and HBT devices have not been widely used in wireless telephony amplifiers and in other applications in which relatively high VSWR conditions can occur.
Accordingly, it is desirable to provide amplifier circuits and techniques that improve the ruggedness of the amplifier without the adverse effects that may have been experienced in the past. Other desirable features and characteristics will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.